Low powered frequency modulation (FM) transmitters are used in some countries for interfacing personal audio devices with radio receivers. The FM transmitter plugs into the headphone jack or proprietary output port of the audio device so that the audio signal can be broadcast over an FM band frequency and received by the radio. One of the main purposes of FM transmitters is to provide a simple and inexpensive means for playing music from an audio device through a car stereo without the need to modify or replace the existing stereo. Nowadays, FM transmitters are being integrated into mobile telephones, personal digital assistants (PDAs), and even laptop computers.
The legal status of unlicensed FM transmission varies around the world. In certain regions it is illegal to transmit on FM frequencies due to the risk of interference with other radio devices. In other regions, however, unlicensed FM transmission is legal provided that local regulations are adhered to. The regulations define technical requirements for FM transmission, but these requirements typically vary from region to region. In Europe and Japan, the maximum output power of an FM transmitter is limited to 50 nW (−43 dBm effective radiated power, ERP), whilst in China and the US, the maximum output is limited to 45 nW (−43.5 dBm) and 250 μV/m@3 m (−47.3 dBm), respectively. Besides output power, the available frequency bands and channel spacing also differ. The available frequency bands are currently 87.6-107.9 MHz in Europe, China and the US, and 76.1-87.5 MHz in Japan, and the channel spacing is 100 kHz in Europe and Japan, and 200 kHz in the US and China.
Manufacturers provide type-approval certificates to demonstrate that an FM transmitter meets the minimum set of regulatory, technical and safety requirements in the country of manufacture, but there is no way for the certification authorities to ensure that local regulations are met when the transmitter is taken from one region to another. As a result, an FM transmitter may be used illegally when the user travels between regions with different transmission requirements. In order to be able to use the same FM transmitter legally in all parts of the world, it is necessary to reconfigure the FM transmission parameters of the FM transmitter according to the region-specific requirements. In some regions, the FM transmitter may even need to be switched off.
At present, one solution involves storing the FM transmission parameters for each region in the device memory and configuring the FM transmitter with the parameters for the current geographical location of the device when FM transmitter is switched on. When the device is moved to a different location, the FM transmitter is then reconfigured with the local parameters to ensure compliance with the requirements of the region in which the device is being used. If certification authorities could certify an FM transmitter for international use, only FM transmitters implementing such dynamic control of parameters would be certified.
In order to determine and track the device location, previous techniques have made use of information obtained from cellular networks. Each cell in the network is uniquely identified within its location area, network and country by means of Cell Global Identification (CGI). When a mobile phone enters a new location and registers to the base station in that cellular location, it has access to the Mobile Country Code (MCC). Therefore, a mobile phone comprising an FM transmitter is able to determine the region in which it is currently located from the MCC of the local base station.
This method for determining and tracking the device location suffers from several disadvantages. Firstly, it is not possible to locate or track the device in areas without network coverage. If there is no network coverage, then the device is not able to determine its current location and obtain the appropriate FM transmission parameters. This can result in two scenarios: either the user proceeds to transmit data and risks breaching the local regulations, or the FM transmitter is automatically disabled by the device until a network connection has been re-established. This limitation could therefore disadvantage a user wishing to transmit music to the FM receiver in his car stereo while driving through mountainous regions or tunnels.
A further problem may occur when the device is located close to a boundary between two regions that require different FM transmission parameters. If the signal from a base station on the opposite side of the boundary is stronger than the signal from any base station on the same side of the boundary, the device may connect to, and receive location data from, the former. In this situation, the FM transmitter may be configured to use parameters which are not valid for use at the current device location.
Also, modern mobile phones often come with additional features (digital camera, MP3 player, games, GPS etc) which can be operated when the phone is in either active mode or flight mode. Active mode refers to the state when the cellular telecommunications circuitry is enabled for use. In contrast, flight mode refers to the state when the cellular telecommunications circuitry (and other transmission circuitry) is disabled from use, and is most often used onboard aircraft to prevent transmitted signals from interfering with the aircraft avionics and ground cell networks. Flight mode can also be used to reduce power consumption and extend battery life during use of the additional features.
Modern mobile phones also allow a user to access the additional features whether a subscriber identity module (SIM) card is in use or not. Operation of the phone without a SIM card automatically disables the telecommunications circuitry. As with flight mode, this mode of operation can be used to reduce power consumption and extend battery life during use of the additional features. The necessity to connect to a telecommunications network in order to configure an FM transmitter with local transmission parameters therefore reduces this ability to extend battery life.
It should not be automatically assumed that the above-mentioned discussion is knowledge already available in the public domain.
US 2009/0017800 discloses a method for configuring an FM radio transmitter on a portable telecommunications device to automatically cease transmission in countries where unlicensed FM transmission is not permitted. The method uses the current cell ID (or GPS, WLAN or other suitable means) to determine the current location of the device. Once the current location is known, the device accesses a country lookup table to determine whether FM transmission is permitted in the country in which the device is currently located. If transmission is allowed, the FM transmitter remains operational, but if transmission is not allowed, the FM transmitter is disabled from use while the device is within that particular country. This method therefore helps to prevent against illegal operation of FM radio transmitters in portable devices. This document does not disclose a method for configuring an FM radio transmitter on a user-portable device to use valid FM transmission parameters corresponding to the current geographical location.
The listing or discussion of a prior-published document or any background in this specification should not necessarily be taken as an acknowledgement that the document or background is part of the state of the art or is common general knowledge. One or more aspects/embodiments of the present disclosure may or may not address one or more of the background issues.